Create <sect3> in the functions-xml section

This is a small change so that a new XMLTABLE sect3 can be added easily
later.

Author: Craig Ringer
Discussion: https://postgr.es/m/CAFj8pRAgfzMD-LoSmnMGybD0WsEznLHWap8DO79+-GTRAPR4qA@mail.gmail.com

doc/src/sgml/func.sgml

@@ -10328,10 +10328,6 @@ SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuf
   <sect2 id="functions-xml-processing">
    <title>Processing XML</title>
 
-   <indexterm>
-    <primary>XPath</primary>
-   </indexterm>
-
    <para>
     To process values of data type <type>xml</type>, PostgreSQL offers
     the functions <function>xpath</function> and
@@ -10339,6 +10335,13 @@ SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuf
     expressions.
    </para>
 
+   <sect3 id="functions-xml-processing-xpath">
+    <title><literal>xpath</literal></title>
+
+    <indexterm>
+     <primary>XPath</primary>
+    </indexterm>
+
 <synopsis>
 <function>xpath</function>(<replaceable>xpath</replaceable>, <replaceable>xml</replaceable> <optional>, <replaceable>nsarray</replaceable></optional>)
 </synopsis>
@@ -10395,6 +10398,10 @@ SELECT xpath('//mydefns:b/text()', '<a xmlns="http://example.com"><b>test</b></a
 (1 row)
 ]]></screen>
     </para>
+   </sect3>
+
+   <sect3 id="functions-xml-processing-xpath-exists">
+    <title><literal>xpath_exists</literal></title>
 
     <indexterm>
      <primary>xpath_exists</primary>
@@ -10425,6 +10432,7 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</m
 (1 row)
 ]]></screen>
     </para>
+   </sect3>
   </sect2>
 
   <sect2 id="functions-xml-mapping">